1. Field of the Invention
The present invention relates to a method of forming a BiCMOS semiconductor chip and, more particularly, to a method of forming a BiCMOS semiconductor chip that increases the betas of the bipolar transistors.
2. Description of the Related Art
A BiCMOS semiconductor chip is a chip that includes bipolar transistors and CMOS circuitry. There are two types of bipolar transistors: p-type (PNP) and n-type (NPN). Similarly, a CMOS circuit includes two types of MOS transistors: p-type (PMOS) and n-type (NMOS). Bipolar transistors have three semiconductor regions: a collector, a base, and an emitter. MOS transistors have four semiconductor regions: a source, a drain, a gate, and a body.
Most of the semiconductor regions are fabricated by forming a patterned photoresist layer over a layer of semiconductor material, and then implanting dopants into the semiconductor material through openings in the patterned photoresist layer. For example, the p+ source and drain regions of a PMOS transistor can be formed by implanting p-type dopants into an n-type body region within the semiconductor material through the openings in a patterned photoresist layer.
One of the most expensive steps in the fabrication of a semiconductor chip is the formation of the patterned photoresist layers. In an effort to reduce costs, the same patterned photoresist layer is commonly used to simultaneously form one or more bipolar semiconductor regions and one or more MOS semiconductor regions. For example, the same patterned photoresist layer can be used to simultaneously form the p+ emitter region of a bipolar transistor, and the p+ source and drain regions of a PMOS transistor.
Although this approach reduces fabrication costs, one drawback of this approach is that the ideal dopant concentration and implant energy used to form a bipolar semiconductor region are typically not the same as the ideal dopant concentration and implant energy used to form a MOS semiconductor region.
For example, the ideal dopant concentration and implant energy used to form a p+ emitter region is typically not the same as the ideal dopant concentration and implant energy used to form the p+ source and drain regions. Emitters that are formed using the dopant concentration and implant energy of the source and drain regions commonly have very low betas. Thus, there is a need for a method of forming a BiCMOS semiconductor chip that increases the betas of the bipolar transistors.